1. Field of the Invention
The present invention relates to a capacitor element which is less likely to break due to heat as compared with prior art capacitor elements. The present invention also relates to a method of making such a capacitor element.
2. Description of the Related Art
FIG. 9 illustrates an example of prior art solid electrolytic capacitor. The capacitor includes a resin package 90 and a capacitor element 91 sealed in the package. The capacitor element 91 includes a porous sintered body 91A, an anode wire 91B partially embedded in the sintered body 91A, and a cathode layer 91C formed on the sintered body 91A. As shown in the figure, the cathode layer 91C is connected to a cathode lead 02, whereas the anode wire 91B is connected to an anode lead 93. The cathode lead 92 and the anode lead 93 include portions 92a, 93a sealed in the package 90 and portions 92b, 93b projecting outward from the package 90. The outer portions 92b, 93b extend downwardly along the side surfaces of the package 90 and then extend horizontally along the bottom surface of the package 90. With such a structure, the capacitor 9 can be surface-mounted on a circuit board by soldering, for example.
In the prior art capacitor 9, the cathode layer 91C has a relatively small thickness in the range of 20-30 xcexcm, for example. However, such a small thickness causes the following problems.
As methods for mounting the capacitor 9 on a circuit board, there exist flow soldering and reflow soldering. In these methods, the capacitor 9 is heated in melting the applied solder material, so that the package 90 and the cathode layer 91C (and other components) undergo thermal expansion. However, since cathode layer 91C differs from the package 90 differs in thermal expansion coefficient, stress is applied to the cathode layer 91C. As a result, the cathode layer 91C which has a relatively small thickness may be broken. Particularly, portions 91c indicated in FIG. 9 (which correspond to corner portions of the sintered body 91A) are further thinner than other portions of the cathode layer 91C. Therefore, there is much possibility that the thin-walled portions 91c break during the flow soldering (or reflow soldering). Further, due to such breakage, the cathode layer 91C becomes likely to be removed from the sintered body 91A.
The above-described defect of the cathode layer 91C is not preferable, because it causes an increase in impedance and dielectric loss of the capacitor element 91. Further, when such defect is serious, the intended capacitor characteristics cannot be obtained at all.
An object of the present invention, which is conceived under the circumstances described above, is to provide a capacitor element capable of preventing the cathode layer from being broken or removed during the soldering.
According to a first aspect of the present invention, there is provided a capacitor element comprising a porous member made of valve metal powder, an anode wire projecting from the porous member, and a cathode layer formed on the porous member. The cathode layer has a thickness of no less than 35 xcexcm. More preferably, the thickness of the cathode layer is no less than 40 xcexcm.
Preferably, the impedance after flow soldering is no more than twice the impedance before the flow soldering. More preferably, the impedance after flow soldering is no more than 1.5 times the impedance before the flow soldering.
Preferably, the cathode layer is formed by heating conductor paste applied on the porous member.
Preferably, the conductor paste contains conductor powder, binder and solvent.
Preferably, the cathode layer is formed by repetitively performing a process which comprises applying conductor paste on the porous member and heating the conductor paste.
According to a second aspect of the present invention, there is provided a capacitor element comprising a porous member made of valve metal powder, an anode wire projecting from the porous member, a cathode layer formed on the porous member, an anode lead connected to the anode wire, a cathode lead connected to the cathode layer, and a resin package sealing the porous member, the anode wire and the cathode layer. The cathode layer has a thickness of no less than 35 xcexcm.
According to a third aspect of the present invention, there is provided a method of making a capacitor element comprising the steps of preparing a porous member of valve metal powder, and heating conductor paste applied on the porous member to form a cathode layer. The cathode layer is formed to have a thickness of no less than 35 xcexcm.
Preferably, the thickness of the cathode layer is controlled by adjusting the viscosity of the conductor paste.
Other objects, features and advantages of the present invention will become clearer from the description of the preferred embodiment given below.